1. Field of the Invention
This invention relates to the production of carbon fibers having a high Young's modulus of elasticity and high tensile strength. More particularly, this invention relates to carbon fibers having a high Young's modulus and high tensile strength produced from pitch which has been transformed, in part, to a liquid crystal or so-called "mesophase" state.
2. Description of the Prior Art
As a result of the rapidly expanding growth of the aircraft, space and missile industries in recent years, a need was created for materials exhibiting a unique and extraordinary combination of physical properties. Thus, materials characterized by high strength and stiffness, and at the same time of light weight, were required for use in such applications as the fabrication of aircraft structures, re-entry vehicles, and space vehicles, as well as in the preparation of marine deep-submergence pressure vessels and like structures. Existing technology was incapable of supplying such materials and the search to satisfy this need centered about the fabrication of composite articles.
One of the most promising materials suggested for use in composite form was high-strength, high-modulus carbon textiles, which were introduced into the market place at the very time this rapid growth in the aircraft, space and missile industries was occurring. Such textiles have been incorporated in both plastic and metal matrices to produce composites having extraordinary high-strength- and high-modulus-to-weight ratios and other exceptional properties. However, the high cost of producing the high-strength, high-modulus carbon textiles employed in such composites has been a major deterrent to their wedespread use, in spite of the remarkable properties exhibited by such composites.
Most high-strength, high-modulus carbon textiles currently available are derived, for the most part, from rayon or acrylic fibers, and are inherently expensive as a result of the high cost of their precursors. In addition to the high cost of the starting materials, the poor carbon yield obtained from such precursors (of the order of 25 to 50 per cent) and the complex processing required to produce satisfactory carbon textiles therefrom materially contribute to the cost of the final product. Thus, carbon textiles prepared from rayon fibers at low temperatures are weak, porous and almost completely disordered in structure, with high modulus and high strength being obtained only by subjecting the fibers to longitudinal stress at high temperatures where the fibers become somewhat plastic. High strength and modulus are generally obtained in carbon textiles derived from acrylic fibers, on the other hand, by the application of longitudinal stress during a lengthy heat stabilization treatment prior to carbonization, generally in an oxygen-containing atmosphere, with the application of stress continued, if desired, during further heat treatment. In both instances, it is necessary to apply stress to the fibers in order to obtain the desired level of modulus and strength. In the case of rayon, the stress is applied at high temperatures in order to align the disordered crystallites present in the fiber parallel to the fiber axis and thereby increase the strength and modulus of the fiber. In the case of acrylic fibers, such as polyacrylonitrile, the precursor is already highly oriented and stress is generally applied prior to carbonization during the heat stabilization treatment in order to maintain this orientation while it is more permanently preserved by the cross-linking which occurs between the fiber molecules during the heat treatment. In either case, the application of stress causes frequent breakage of the fibers during processing, requires additional processing apparatus, and materially contributes to the cost of the fiber.
Rayon and acrylic fibers are not only expensive and difficult to process to carbon textiles, but they are also "non-graphitizing" materials incapable of being substantially converted by heat treatment to the three-dimensional crystalline structure characteristic of polycrystalline graphite. While carbon produced from most carbonaceous precursors can to some degree be transformed by further heat treatment from the less ordered structure of the carbonized product to a structure which more nearly resembles the three-dimensional crystalline structure characteristic of polycrystalline graphite, only carbon produced from certain so-called "graphitizable" or "graphitizing" materials, such as petroleum coke, are capable of full development of a graphite structure and graphitic-like properties associated therewith, such as high density and low electrical resistance. Rayon and acrylic fibers, as is characteristic of materials which pyrolyze to a char without melting, are among those materials which are incapable of forming large crystallites having a high degree of three-dimensional order. Despite this, fibers produced by pyrolysis of such materials have traditionally been classified as carbonized or graphitized on the basis of their elemental carbon content or the temperature to which they have been heated. Thus, for example, Schmidt and Jones have classified fibers prepared at temperatures ranging from 1300.degree. F. to 1700.degree. F (704.degree. C. to 927.degree. C.) as partially carbonized or carbonized, while fibers processed at 4900.degree. F. to 5400.degree. F. (2704.degree. C. to 2982.degree. C.) are classified as graphite; similarly, fibers having an elemental carbon content up to 90 weight percent are classified as "partially carbonized", while fibers having an elemental carbon content in the range of 91 to 98 weight percent are classified as carbonized, and fibers having an elemental carbon content in excess of 98 weight percent are classified as graphite (Schmidt, D. L. and Jones, W. C., "Carbon-Base Fiber Reinforced Plastics," AFML, WPAFB, Dayton, Ohio. ASD-TDR-62-635, August, 1962). Such classification methods, however, fail to take into consideration the actual crystal structure of the fiber. Thus, for example, by such methods of classification, a graphitized fiber could be one processed at a very high temperature or one having a very high elemental carbon content, even though prepared from a non-graphitizing precursor and substantially devoid of the three-dimensional crystalline structure characteristic of polycrystalline graphite.
While high-modulus, high strength carbon fibers prepared by processing rayon and acrylic fibers to temperatures of from 2500.degree. C. to 300.degree. C. and higher, develop certain incipient graphitic-like properties with increasing temperature, such as high density, high carbon content, and low electrical resistivity, the fibers, as aforementioned, are incapable of full development of the three-dimensional ordered structure of polycrystalline graphite. As the fibers are heated to a temperature sufficiently high to produce a substantially all-carbon fiber, e.g., to a temperature of about 1000.degree. C., planes of carbon atoms arranged in polynuclear aromatic rings and stacked parallel to each other gradually develop within the fiber. On further heating above about 1000.degree. C., these stacks, or crystallites, continue to grow in size, either by coalescence with other crystallites or by the incorporation of surrounding disorganized carbon atoms, and on heating to so-called graphitizing temperatures, the layer planes within the crystallites begin to rearrange themselves somewhat by mutual rotation and shifting. However, both crystallite growth and rotation of the layer planes within each crystallite is minimal, and the resulting crystallites are both small and turbostratic, i.e., although the layer planes within the crystallites are all essentially parallel to each other, extensive rotational misalignment of these layers relative to each other exists. While the application of longitudinal stress to the fibers (at high temperatures in the case of rayon or during heat stabilization in the case of acrylic fibers) produces some ordering in the fiber structure by aligning these crystallites parallel to the longitudinal fiber axis, each crystallite still remains turbostratic and essentially devoid of the three-dimensional order of polycrystalline graphite, even after heating to high temperatures. The preferred orientation of the crystallites parallel to the longitudinal fiber axis imparts high modulus and strength to the fibers, but the failure of the carbon planes within each crystallite to align themselves relative to each other prevents the fibers from developing truly graphitic properties, e.g., high thermal and electrical conductivity.
The high degree of preferred orientation of the fiber crystallites of high-modulus, high-strength carbon fibers prepared by processing rayon and acrylic fibers to temperatures of from 2500.degree. C. to 3000.degree. C., and higher, is clearly established by the short arcs which constitute the (00l) bands of the X-ray diffraction pattern of these fibers. The turbostraticity of these crystallites, i.e., the misalignment of the parallel layers within the crystallite relative to each other, is evident from the absence of the (112) cross lattice line in the pattern and the lack of resolution of the broad (10) diffraction band into two distinct lines, (100) and (101). The lack of three-dimensional order within the crystallites is further indicated by the relatively high interlayer spacing (d) of the layer planes, which has been shown to exceed 3.40 A in the case of fibers prepared from polyacrylonitrile or rayon. This measurement is calculated from the distance between the corresponding (00l) lines of the X-ray diffraction pattern and has been related by R. E. Franklin to the proportion of disoriented layers, or disorientation parameter p of carbon (R. E. Franklin, Acta Cryst., 4, 253, 1951)..sup.(1) Based on the relationship shown by Franklin, the disorientation parameter p of fibers prepared from either polyacrylonitrile or rayon exceeds 0.7. It is considered that carbon which after undergoing heat treatment to 3000.degree. C. has an interlayer spacing d.sub.002 greater than 3.40 A or a disorientation parameter p greater than 0.7 is a non-graphitizing carbon, while carbon which after undergoing heat treatment to 3000.degree. C. has an interlayer spacing d.sub.002 less than 3.37 A or a disorientation parameter of less than 0.4 is well-graphitizing or graphitic (see, e.g. U. K. patent No. 1,220,482). FNT (1) The proportion of disoriented layers p was calculated from the (112) line on the assumption of a randon distribution of orientations and disorientations. This measurement was then related to the interlayer spacing d.sub.002 assuming there exist only three interlayer spacings, 3.354 A at an orientation or at a disorientation isolated between two orientations, 3.399 A at the first disorientation on either side of an oriented group, and 3.440 A at all other disorientations.
In addition to having an interlayer spacing greater than about 3.40 A and a disorientation parameter greater than about 0.7, the crystallites of high-modulus, high-strength carbon fibers prepared by processing rayon and acrylic fibers to temperatures of from 2500.degree. C. to 3000.degree. C., and higher, are considered to be non-graphitic in that they are incapable of developing a crystallite size characteristic of graphitic carbon, i.e., a layer size (L.sub.a) and a stack height (L.sub.c) in excess of 500 A. Thus, the apparent layer size (L.sub.a) of the crystallites of these materials does not exceed 200 A, while the apparent stack height (L.sub.c) does not exceed 100 A. Because of their small size, these crystallites are incapable of being detected by conventional polarized light microscopy techniques at a magnification of 1000..sup.(2) FNT (2) The maximum resolving power of a standard polarized light microscope having a magnification factor of 1000 is only a few tenths of a micron (1 micron = 10,000 A). Thus, crystallites having dimensions of 1000 A or less cannot be detected by this technique.
While Jackson and Marjoram (Jackson, P. W. and Marjoram, J. R., Nature, Vol. 218, pages 83-84, Apr. 6, 1968) have reported that carbonized fibers prepared by controlled pyrolysis of polymer fiber up to 1000.degree. C. and graphitized fibers prepared by further treatment up to 2700.degree. C. may be recrystallized to produce graphitized fibers having extensive three-dimensional order and a crystallite size of 500 A by coating the fibers with nickel and heating above 1000.degree. C. for 24 hours, such recrystallization is accompanied by a drastic reduction in the strength of the fibers. The weakened fibers are, of course, difficult to separate from their nickel coating, prohibitively expensive to make, and unsuitable for preparing composites having high-strength- and high-modulus-to-weight ratios.
In addition to rayon and acrylic fibers, various natural and synthetic pitches have been suggested as precursor materials for carbon textiles. Although these materials are suitable for the production of carbon fibers because of their high carbon content and ability to form spinnable melts, the thermoplastic nature of pitch makes it impossible to carbonize fibers drawn therefrom without first thermosetting the fibers to ensure preservation of the filament shape during carbonization. Thermosetting is generally accomplished by extended heating in air or other oxygen-containing atmosphere until the fibers are rendered infusible. However, such treatment not only renders the fibers infusible but also inhibits crystallite growth and alignment during subsequent heat treatment and prevents the fibers from developing a graphitic structure. Consequently, the carbon fibers produced are composed of small turbostratic crystallites which do not possess the high degree of crystallite orientation along the fiber axis ordinarily associated with high fiber modulus.
The first publication on the subject of producing carbon fibers from pitch (Otani, S., "On the Carbon Fiber from the Molten Pyrolysis Products," Carbon 3, 31-38, 1965) did not deal with commercial pitches, such as coal tar pitch or petroleum pitch, but with a specially prepared pitch produced by pyrolyzing polyvinyl chloride at a temperature of about 400.degree. C. - 415.degree. C. for 30 minutes or more in a nitrogen atmosphere. This method proposed making carbon fibers from such pitch by melt spinning the pitch to produce a fiber, oxidizing the fiber with ozone below 70.degree. C. and/or in air up to 260.degree. C. to produce an infusible fiber, and subsequently carbonizing the fiber to a temperature of 500.degree. C. to 1350.degree. C. in a nitrogen atmosphere. Although the fibers prepared in this manner were composed of glassy carbon, tensile strengths of up to about 18 .times. 10.sup.6 g/cm.sup.2 (256,000 psi.) were reported. However, the highest modulus obtained for such fibers was less than 5 .times. 10.sup.8 g/cm.sup.2 (8 .times. 10.sup.6 psi.), evidently due to the lack of crystallite orientation within the fiber. When the residual tarry material formed as a by-product in the production of benzylchloride by the reaction of chlorine and toluene was employed as starting material, almost identical fibers were said to have been obtained.
Later, the preparation and properties of carbon fibers spun from petroleum asphalt and coal-tar pitch was discussed by Otani (Otani, S., Yamada, K., Koitabashi, T., and Yokoyama, A., "On the Raw Materials of MP Carbon Fiber," Carbon 4, 425-432, 1966). These materials were spun into fibers at temperatures between 200.degree. C. and 370.degree. C. after being first dry distilled (by bubbling nitrogen gas through the pitch) at about 380.degree. C. for 60 minutes and then vacuum distilled at 380.degree. C., or less, for 60-80 minutes. In the case of the coal tar pitch, additional heating at 280.degree. C. under nitrogen after adding dicumilperoxide was necessary to improve spinnability at high speeds. The spun fibers were rendered infusible by oxidizing in ozone at 60.degree. C. to 70.degree. C. and then in air to 260.degree. C., and were subsequently carbonized by heating to 1000.degree. C. in a nitrogen atmosphere. The properties of fibers drawn from petroleum asphalt were similar to those of fibers which had been prepared from polyvinyl chloride pitch, but fibers prepared from coal tar pitch were lower in strength and more difficult to spin. Fibers prepared from mixtures of petroleum asphalt and coal tar pitch more nearly resembled fibers prepared from petroleum asphalt than fibers prepared from coal tar pitch.
More recently, Hawthorne et al. reported that the tensile strength and Young's modulus of carbon fibers produced from petroleum asphalt and other pitches in a manner similar to that employed by Otani et al. may be raised from 250 .times. 10.sup.3 psi. and 3-7 .times. 10.sup.6 psi., respectively, to 375 .times. 10.sup.3 psi. and 70 .times. 10.sup.6 psi., respectively, by elongating the fibers at a temperature of from 2000.degree. C. to 2800.degree. C. (Hawthorne, H. M., Baker, C., Bentall, R. H., and Linger, K. R., "High Strength, High Modulus Graphite Fibres from Pitch," Nature 227, 946-947, Aug. 29, 1970). The structure of the fibers produced in this manner were said to resemble the structure previously observed in rayon and polyacrylonitrile graphite fibers. As with fibers derived from these earlier precursors, however, although the application of longitudinal stress to the fibers produces a high degree of orientation of the fiber crystallites parallel to the longitudinal fiber axis, each crystallite remains turbostratic and essentially devoid of the three-dimensional order characteristic of polycrystalline graphite.
In a still later report, Hawthorne more fully discussed the structure of fibers made by high temperature stretching of the glassy carbon fibers derived from pitches and like precursors (Hawthorne, H. M., "Structure and Properties of Strain-Graphitized Glassy Carbon Fibres," International Conference on Carbon Fibres, their composites and Applications, The Plastics Institute, Paper No. 13, 13/1-13/13, London, 1971). The X-ray diffraction characteristics of these fibers were said to be generally similar to polyacrylonitrile- and rayon-based fibers in that no reflections other than (001) lines and (hk) bands are present, consistent with the turbostratic nature of these fibers. The fiber crystallites were shown to have a large d-spacing (.gtoreq. 3.40 A) and small apparent crystallite size (L.sub.a .ltoreq. 136 A; L.sub.c .ltoreq. 145 A), which are characteristic of glassy carbons. Fibrils having widths of up to 300 A and granular domains 800-900 A in diameter were indicated.
Otani et al. have further reported that carbon fibers having a high degree of preferred orientation of carbon crystallites parallel to the fiber axis can be obtained from pitch materials not only by applying stress at high temperatures to fibers drawn from such materials, in the manner of Hawthorne et al., but also, without the application of stress, from a pitch which possesses highly-oriented molecules that is prepared from tetrabenzophenazine (Otani, S., Kokubo, Y., Koitabashi, T., "The Preparation of Highly-oriented Carbon Fiber from Pitch Material", Bulletin of the Chemical Society of Japan, 43, 3291-3292, October, 1970). However, the method of preparing such fibers was not disclosed. Although fibers prepared from such pitch were reported to be highly oriented, such fibers were not indicated to have a graphitic-like structure or to in any way differ from highly oriented carbon fibers earlier prepared from pitch precursors by the application of stress at high temperatures.
Thus, although it is well known that pitch materials can be transformed by heat treatment at elevated temperatures from an isotropic structure to one containing domains of highly oriented molecules (Brooks, J. D., and Taylor, G. H., "The Formation of Some Graphitizing Carbons," Chemistry and Physics of Carbon, Vol. 4, Marcel Dekker, Inc., New York, 1968, pp. 243-268; White, J. R., Guthrie, G. L., and Gardner, J. O., "Mesophase Microstructures in Carbonized Coal Tar Pitch," Carbon 5, 517, 1968; and Dubois, J., Agache, C., and White, J. L., "The Carbonaceous Mesophase Formed in the Pyrolysis of Graphitizable Organic Materials," Metallography 3, 337-369, 1970), no method for converting such materials into carbon fibers having the three-dimensional crystalline structure characteristic of polycrystalline graphite has been reported. Carbon fibers having such structures are still unknown, and, to date, all high modulus, high strength carbon fibers derived from pitch precursors, whether by high temperature stretching or directly from high oriented pitch precursors in the absence of stress, differ little in structure from high modulus, high strength carbon fibers produced from rayon or acrylic precursors. Although all such fibers, regardless of precursor, are characterized by the presence of carbon crystallites preferentially aligned parallel to the fiber axis, none possesses the three-dimensional order characteristic of polycrystalline graphite.